Primary analysis of MUKDEN 01: A multicenter, single-arm, prospective, phase 2 study of neoadjuvant treatment with pyrotinib and letrozole plus dalpiciclib in triple-positive breast cancer.

588 Background: Despite the use of multiple lines of targeted therapy has revolutionized treatment for HER2-positive breast cancer, these methods still have limited efficacy for triple-positive breast cancer (TPBC), which calls for persistent exploration for optimized treatment strategy. This MUKDEN-01 prospective trial aimed to evaluate the efficacy of oral, chemo-sparing neoadjuvant therapy with pyrotinib, letrozole and dalpiciclib, which also meet the need for treatment convenience under COVID-19 pandemic, for patients with TPBC. Methods: The MUKDEN 01 was an investigator-initiated, multicentre, single arm, prospective phase II trial, which was performed at twelve hospitals in China(NCT04486911). Treatment-naïve patients with stage II-III tumors that according to the AJCC 8 th edition criteria were eligible. Patients were treated with each cycle of 4 weeks with oral administration of pyrotinib 320 mg, and letrozole 2.5mg once daily for 4 weeks, and dalpiciclib 125 mg once daily for three weeks, followed by one week off, for five cycles. The primary endpoint was pathological complete response (pCR) in the breast and axilla (ypT0/is ypN0). Secondary endpoints included pCR in the breast (ypT0/is). residual cancer burden (RCB) score, Ki67 index change at surgery compared with baseline, and safety. Safety was analyzed in all patients, who received treatment. The study is still ongoing, and the enrollment has been completed. Results: Between June 20, 2020 and Sep. 6, 2021, 68 patients were screened for eligibility and 61 patients were recruited into this first stage of study. After surgery, 18 (29.5%, 95% CI 18.5-42.6) out of 61 patients achieving tpCR(ypT0/is ypN0), 21 (34.4%, 95% CI 22.7-47.7) patients achieved bpCR(ypT0/is). The patients with excellent pathologic response (RCB 0-1) to the combined therapy accounted for 54.1% (33/61, 95% CI 40.9-66.9). Mean Ki67 expression was reduced from 38.7% (95%CI: 31.3-46.0) at baseline to 19.3% (95% CI:13.6-25.0; p=0.0001) in the surgical samples. The most frequent grade 3 AE were neutropenia (35 [57%]), leukopenia (13 [21%]), diarrhea (9 [15%]) and oral mucositis (4 [7%]). There were five grade 4 neutropenia (8%) and one grade 4 increased AST (2%), but without other SAE and death throughout the study. Conclusions: Neoadjuvant therapy with pyrotinib, letrozole and dalpiciclib yielded a pCR rate comparable to standard chemotherapy plus dual HER2 blockade in TPBC patients. The combined therapy was also well-tolerated and provided a chemo-sparing neoadjuvant approach for TPBC patients. To our knowledge, this is the first study to evaluate the therapeutic efficacy of a chemo-free neoadjuvant treatment with HER2 TKI pyrotinib and letrozole plus CDK4/6 inhibitor dalpiciclib for TPBC patients. Further validation in a large-scale randomized controlled trial is warranted. Clinical trial information: NCT04486911.